This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The aim of this project is to examine the influence of the geometry of extracellular space on diffusion of neurotransmitter within specialized synaptic structures called cerebellar glomeruli. We focus on the cerebellar glomeruli since their unique physiology allows a non-classical type of chemical neurotransmission whereby chemicals released at a given synaptic contact are able to reach nearby synaptic contacts. This allows us to ask whether release sites within a glomerulus are arranged with respect to each other and with respect to various structural features in such a way as to yield a different effective diffusion coefficient than one obtained with a random distribution of release sites. In other words are diffusion paths between actual release sites uniquely set up to either facilitate or hamper diffusion of neurotransmitter between them? To address this question we will simulate spillover diffusion in models of cerebellar glomeruli whose geometry is based on electron tomographic reconstructions. This proposal is the second of two part proposal setup. The 1st portion was sent out in the middle of September 2007, as a DAC proposal for 30000 service units. This proposal requests the remaining 114000 service units.